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Abstract
In intact cyclic ewes intrauterine infusion of conceptus secretory proteins results in the suppression of both endometrial oxytocin receptor concentrations and oxytocin-induced prostaglandin F2α release. However, similar infusion in progesterone-treated ovariectomized ewes, while suppressing endometrial oxytocin receptors, does not fully inhibit oxytocin-induced prostaglandin F2α release. To examine whether this anomaly resulted from an inadequate simulation of the luteal phase in the ovariectomized ewe treated with progesterone alone, the effects of additional treatment with two other ovarian hormones, oestradiol-17β and oxytocin, was investigated. Rather than permitting conceptus secretory protein to successfully inhibit oxytocin-induced prostaglandin F2α release, treatment with oestradiol-17β in addition to progesterone actually resulted in an advancement in the timing of release. However, treatment with oxytocin, alone or in combination with oestradiol, permitted the full inhibition of oxytocin-induced prostaglandin F2α release. To confirm that this effect did not result from the action of oxytocin alone, independently of the action of conceptus secretory protein, a second experiment was undertaken using a similar protocol but without the infusion of conceptus secretory protein. In this situation, oxytocin-induced prostaglandin F2α release was only partially inhibited suggesting that both luteal oxytocin and conceptus secretory proteins are necessary to facilitate the full inhibition of luteolysis during early pregnancy in the ewe.
Journal of Endocrinology (1996) 150, 473–478
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ABSTRACT
Transection of the ventral central grey and surrounding midbrain tegmentum in anaesthetized lactating rabbits caused repeated milk ejections which, on comparison with the effects of i.v. infusions of synthetic oxytocin, synthetic arginine-vasopressin or a mixture of the two, were attributed to continuous release of 1·25–2·5 mu. oxytocin/min, although it is not known whether lesser amounts of vasopressin might also have been released. It is suggested that the ascending midbrain reticular formation, which is known to project rostrally through this region, controls the central inhibition of oxytocin release via the previously described septo-hippocampo-subicular route to the hypothalamus.
J. Endocr. (1986) 109, 405–409
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SUMMARY
Two peptides with uterotonic activity have been isolated from the pituitary gland of a holocephalian elasmobranch fish: Hydrolagus collei. One of them had an amino acid composition compatible with that of oxytocin itself, and also had the pharmacological properties of this hormone. The other peptide which was present in much smaller amounts was basic by chromatography and had the pharmacological characteristics of [8-arginine]-oxytocin. It was not completely purified because of the small amount available, but its amino acid composition was in accord with that of vasotocin. The implications of the presence of oxytocin in such a primitive fish on the phylogeny, and hence probably the evolution, of neurohypophysial hormones are discussed.
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ABSTRACT
Injections and infusions of oxytocin into conscious dogs caused an increase in plasma concentrations of glucose, insulin and glucagon. When blood glucose was clamped at a raised level the injection of oxytocin still increased insulin and glucagon concentrations in plasma. Infusion of somatostatin suppressed plasma concentrations of glucagon and insulin but did not prevent oxytocin-induced increments in blood glucose. Injection of oxytocin still caused a marked release of glucagon, whereas the insulin response was greatly diminished. When endogenous insulin and glucagon secretion was suppressed by infusion of somatostatin and glucose levels were stabilized by concomitant infusions of glucagon and insulin, injections of oxytocin did not alter blood glucose concentrations. It is concluded that the increase in blood glucose following the administration of oxytocin is secondary to the release of glucagon and that oxytocin exerts a direct stimulatory effect on glucagon and possibly insulin secretion.
J. Endocr. (1986) 108, 293–298
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The use of agarose-bound neurophysin for the extraction of oxytocin from biological fluids is described. Oxytocin can be extracted from plasma, urine and cerebrospinal fluid with a high rate of recovery and samples varying widely in volume and oxytocin concentration can be tested by the method.
Columns can be used to extract and concentrate dilute samples, or to help identify small amounts of neurohypophysial hormones by affinity chromatography. The oxytocin can be eluted from the column directly into the buffer used for subsequent bioassay. The composition of the final extract is constant and independent of the composition of the sample. The specificity of the binding is high. It is suggested that the method has many advantages over others in current use.
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Netherlands Central Institute for Brain Research, Ijdijk 28, Amsterdam-0, The Netherlands
(Received 3 July 1975)
The 'classical' view on the distribution of oxytocin and vasopressin producing cells suggests that the paraventricular nucleus (PVN) is predominantly or entirely responsible for oxytocin production and the supraoptic nucleus (SON) synthesizes mainly vasopressin. However, recent hormone assays and electrophysiological studies indicate the presence of each hormone in both nuclei (for references see Burford, Dyball, Moss & Pickering, 1974). We report an immunofluorescence study in the SON and the magnocellular part of the PVN (Bodian & Maren, 1951) using antibodies to oxytocin (produced in our laboratory) and to vasopressin (produced by Drs Hollemans, Schellekens and Touber), purified by absorption with arginine-vasopressin and oxytocin respectively (Swaab & Pool, 1975).
Frontal cryostat serial sections of glyoxal-prefixed hypothalami from five male Wistar rats weighing 200 g were studied. Out of each group of six sections, the first was
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SUMMARY
Oxytocin levels in pregnant and parturient guinea-pigs were studied by means of a sensitive and specific radioimmunoassay. Oxytocin was released from the maternal pituitary in substantial amount only during the expulsive phase of labour, when the mean concentration in carotid arterial blood in five animals was 503 pg/ml plasma (range 96–2900 pg/ml). Oxytocin was not found in the plasma of the first born at the moment of birth, but was usually detected in amounts ranging from 96 to 455 pg/ml in those born subsequently. The mean half-time of oxytocin in the maternal circulation during late pregnancy was 62 ± 7·5 (s.e.m., n = 5) s. In-vivo experiments showed that the placenta was permeable to oxytocin in both directions.
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Oxytocin disappears rapidly from the circulation; half-lives in the circulation for the intravenously injected hormone of 40 sec. to 4 min. have been reported in a variety of species. In connexion with the interpretation of blood levels of oxytocin during suckling (Folley & Knaggs, 1966) the following experiments were carried out to determine the half-life of exogenous oxytocin in the circulation of the sow. Although the results are limited they may be worthy of report since no previous information for the sow is available.
Two experiments were performed on an adult sow (no. 2) weighing 102 kg. which had just lost a premature litter. In the first, the whole experiment was carried out while the sow was maintained under the anaesthetic (cyclopropane/oxygen) given for the insertion of a jugular cannula (for cannulation technique see Folley & Knaggs, 1966). One i.u. oxytocin (Pitocin, Parke, Davis and Co.) in 1 ml. 0·9%
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Although the release of oxytocin from the neurohypophysis has been demonstrated both after physiological stimuli such as suckling (Fitzpatrick, 1961) and after administration of pharmacological agents such as nicotine (Bisset & Walker, 1953), very little work appears to have been carried out to investigate how quickly oxytocin is repleted at the posterior pituitary gland after release. In the present investigation the oxytocin content of the pituitary gland was determined at 2, 10, 30 and 120 min after intravenous administration of nicotine to rats. The effect of previous administration of pheniramine, an antihistamine, on the oxytocin-releasing action of nicotine was studied in a further series of experiments.
Male albino rats (150–200 g) were used. Nicotine bitartarate at a dose of 1 mg/kg was injected slowly into the tail vein of the rat. Groups of rats were killed by decapitation at 2, 10, 30 and 120 min after the administration of nicotine.
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It has been suggested that in certain circumstances oxytocin may stimulate the release of prolactin in the rat (Benson & Folley, 1956) and goat (Bryant, Greenwood & Linzell, 1968). In order to test this hypothesis in a physiological situation the changes in blood levels of oxytocin and prolactin were determined in a series of samples taken during parturition in the goat, where oxytocin levels are known to be high (Folley & Knaggs, 1965; McNeilly, Martin, Chard & Hart, 1972).
The cannulation and blood sampling technique has been described previously (McNeilly et al. 1972). Jugular blood samples were taken continuously during the whole of labour in six pedigree British Saanen goats and all plasma samples were stored at −20 °C until assay. Oxytocin was
